Operational amplifiers have been widely used in the electronics industry for their many excellent circuit characteristics including high open loop gain, high input impedance, and low output impedance. General applications of the operational amplifier include circuit configurations such as voltage and current amplifiers, differentiators and integrators, active filters, oscillators, and analog to digital and digital to analog converters. To realize these different circuit configurations, operational amplifiers are used in conjunction with positive and or negative feedback in combination with passive and or active elements.
An operational amplifier is also widely used to function as a voltage comparator, wherein typically, a reference signal is applied to the inverting input and the voltage to be compared is applied to the noninverting input. If the voltage to be compared is greater than the reference signal, the output of the comparator equals the magnitude of the positive supply voltage, and if the voltage to be compared is less than the reference signal, the output of the comparator equals the magnitude of negative or ground supply voltage. An inverted voltage comparator may be provided by simply transposing the signals at the inverting and noninverting inputs. Using the operational amplifier as a voltage comparator requires no external components or feedback, and its output has only two states of high and low.
The operational amplifier utilized in the realization of a voltage comparator may be manufactured in bipolar or Complementary Metal Oxide Semiconductor (CMOS) technology or some combination thereof. The CMOS implementation is desireable for its low power consumption characteristic. There are several problems generally associated with the operational amplifier when used in the voltage comparator configuration. The first problem is associated with the range of the voltages that may be compared. Typically, CMOS operational amplifiers cannot accurately compare the two input voltages over the entire range of the positive and ground supply voltages. The second problem associated with all operational amplifiers is that of the offset error voltage. This type of error appears as deviations in the expected output due to imbalances in the input stage which is further a result of statistical deviations in the parameters between transistors. The offset error voltage can be corrected but requires additional external connections.
Another problem known as chatter is also associated with voltage comparators and results in the output switching between states very quickly due to the difficulty of differentiating the two input voltages when the difference in their magnitudes is very small. This may be corrected by incorporating hysteresis in the voltage comparator. This, however, requires feedback utilizing high value resistors, designed for a specific switching point thus reducing flexibility, and hence consuming significant silicon area.
Thus, what is needed is a monolithically integrated voltage comparator circuit that compares two input voltages which may vary over the entire range of the supply voltages without requiring offset error voltage correction.